New book examines the mixed-up senses of synaesthesia

Do letters have colours? Does music have flavour? Can you see shapes when you think of numbers? For one in twenty people, the answer to one or more of these questions is "Yes!"

A new book by a University of Sussex psychologist sets out to explore and explain the phenomenon of linking separate sensations, known as synaesthesia. Dr Jamie Ward, the world's leading expert in this field, is the author of The Frog Who Croaked Blue (Routledge), a book that looks not just at the experience of synaesthetes, but poses questions about its usefulness, its purpose and its potential.

Dr Ward, who heads a research group at the University of Sussex on synaesthesia, says that the traditional view that there are five senses - vision, hearing, touch, tastes and smell - is up for challenge. "The basic senses can be broken down into several dimensions. For instance, in the 'vision' domain, some synaesthetes might experience colour, whereas others might experience shapes, and yet others might experience movements." Another difficulty with this formulation, he says, is that synaesthesia can be triggered by things which are not strictly sensory, such as numbers, letters, words, and names. "The number of potentially different forms of synaesthesia is likely to be very big indeed."

The title of Dr Ward's book is taken from a 1922 study of the youngest-known synaesthete, a three-year-old boy, Edgar Curtis, in whom noise produced colour. Other more famous synaethetes include artists David Hockney and Wassiliy Kandinsky, and the philosopher Ludwig Wittgenstein.

Although research has continued for several decades, Dr Ward said there was still much not yet understood about the subject. There was no evidence that synaesthetes differed in their IQ level compared to the rest of the population, but it had been shown that they had a better memory for things in which their synaesthesia colours were involved. There was also some evidence that the brains of synaesthetes responded differently to stimuli. "When certain synaesthetes hear spoken words then the parts of the brain normally dedicated to colour are used," points out Dr Ward. "This suggests that parts of the brain that are normally used to process colour derived from vision are used instead to process colour derived from speech."

It is also a possibility that we are all born as synaesthetes, but that most of us lose the ability at some stage in our development. "We know that it can be hereditary - up to 40 per cent of synaesthetes know that a close family member has the same ability," says Dr Ward. "But often synaesthetes don't discuss it. They either don't realise they have something that others don't have, or they fear that others will be dismissive."